Following the pyrolysis of oil shale to extract the volatile components, such as shale oil and hydrocarbon gases, a solid material remains which is referred to as "retorted oil shale". This material contains residual carbonaceous material which may be burned to yield heat energy. The heat recovered from the carbonaceous residue may be used to supply heat for the pyrolysis of the oil shale during the retorting process.
The inorganic residue that remains after the combustion of the carbonaceous material present in the retorted shale is called "burned shale". This material is recycled in some retorting processes as "heat transfer material", i.e., the hot burned shale from the combustion is mixed with raw oil shale and the heat provided is used for retorting the raw shale. In U.S. Pat. No. 4,199,432 a process is described in which the oil shale is retorted in a downward moving bed containing a mixture of recycled hot burned shale (used as a heat transfer material) and particulate raw oil shale. A countercurrent flow of gas passes upward through the bed removing the product vapors and pneumatically entraining the finer particulate fraction of the oil shale. The fine particles and product vapors are drawn off the top of the retorting vessel, and the fine shale particles are removed from the gas stream in a cyclone. Processes such as this present some problems in designing an efficient combustor for burning the retorted oil shale which is recycled as the heat transfer material.
During combustion of the carbonaceous residue in the retorted oil shale to produce heat, the physical integrity of the shale particles is changed and a substantial amount of fine grained burned shale is produced which is not suitable for use as recycled heat transfer particles. Therefore, it is necessary to separate this fine material prior to recycling the coarser grained particles.
In process schemes using a liftpipe combustor to burn the residual carbonaceous material in the retorted oil shale, sufficient residence time is required to complete combustion and to assure a thermal equilibrium between the hot burning particles and the cooler recycle particles. Typically, a minimum residence time of 2 to 3 seconds in the combustion zone is required. If the fine shale and coarse shale particles are combusted in a liftpipe, the pipe must be of sufficient length to provide adequate residence time for all particles.
The present invention is advantageous for efficiently burning particulate retorted oil shale where the fine grained material and the coarser grained material are separated prior to combustion and the burned shale serves as heat transfer material in the retorting process.